The invention relates to a device for osteosynthesis having several longitudinally spaced elongated holes for the reception of bone screws.
The simplest osteosynthesis plate is constituted by an elongated, flat, stiff plate element having one or two rows of circular holes, through which the bone screws are adapted to be screwed into the bone, in order to fixedly anchor the plate to the bone on both sides of the fracture surface. Such a plate, however, suffers from the disadvantage that the bores in the bone must be precisely set by the surgeon so that the plate may be neatly fitted. Otherwise the danger exists that the fracture surfaces, for example, may not lie one against another in a fixed relationship.
It has also become known in connection with osteosynthesis plates to use clamping devices, in order to build up a sufficient compression pressure between the fracture segments, prior to finally fixing the plate.
It has furthermore become known in connection with osteosynthesis plates to utilize elongated holes (French patent Letter No. 2 517 536 or German publication letter No. 2 806 414). Such elongated holes may be formed conically at the ends so that with the aid of bone screws the head of which is formed conically or crowned at the underside thereof, a pulling force may be exerted on the plate and, thus, on the fracture segments. Such osteosynthesis plates are also called compression plates. It is intended to generate with them a compression pressure in the fracture surface. What is disadvantageous with such plates is the fact that here as well the bores must be set very accurately by the surgeon, in order to be able to fully utilize the anyway relatively small path of adjustment. If, with respect to one elongated hole this path of adjustment is obtained only in part or not at all, it is no longer possible also with respect to the other elongated holes to generate any tension.
During the healing process, a so-called sintering of the fracture will occur, which may entail a shortening of the bone in this region. In order to make sure that the fixation with the aid of the osteosynthesis plate may follow this, the screws must move relative to the plate in the elongated holes. Between the screw head and the hole, however, there is essentially only a point contact taking place. By virtue of the high surface pressure at the contact points the screw head digs itself more and more into the material of the plate and causes an extraordinarily high coefficient of frictional adhesion which practically prevents the relative movement as described above being obtained with the normally occurring forces. Besides, it is not possible in this manner to apply to the bone screws a given torque in a manner to be capable of reproduction. The known osteosynthesis plates, in addition, are provided with elongated holes over the entire length thereof. Even if a relative movement is possible, a sort of floating retention or bearing of the plate will occur, finally abolishing the desired effect of fixation.